Circadian rhythms and environmental lighting regulate a number of endocrine and behavioral functions. Dispersed chick pineal cells remain rhythmic and responsive to light in culture. Photosensitivity appears to reside in the same cells as melatonin production. The mechanisms of phototransduction appear to differ from those in retinal rod cells. Light, membrane potential, norepinephrine, cyclic AMP, and calcium channels regulate melatonin rhythms in these cells.